1. Field of the Invention
The present invention relates to a developing apparatus used for developing an electrostatic latent image that is formed on an electrostatic latent image bearing member in an image forming apparatus such as a copier, a printer, a facsimile and the like.
2. Description of Related Art
Conventionally, an electrophotographic image forming apparatus, which optically scans an original image portion that is supported on an outer circumferential surface of a uniformly electrified photosensitive drum (an electrostatic latent image bearing member) so as to form an electrostatic latent image, and converts the electrostatic latent image into a visible image by using a toner, that can be a colored resin, is known. Such an image forming apparatus is capable of forming an image at a high speed, and thus has been used widely for digital printers, copiers and the like.
In recent years, there have been increasing demands on such apparatuses, particularly for forming color images. As electrophotographic image forming apparatuses, apparatuses for forming full color images composed of toner images of four colors: yellow (Y); magenta (M); cyan (C); and black (Bk) also have been available. In particular, tandem type image forming apparatuses that are advantageous for high-speed printing have been widely used, and they are becoming more and more mainstream.
This type of tandem type image forming apparatus includes image forming units for each of four colors that are disposed in parallel, and thus gives rise to a problem that the size of the apparatus increases. In order to solve this problem, a configuration in which a spacing or a pitch between photosensitive drums that are adjacent to each other is shortened in an image forming apparatus of a non-magnetic one-component developing system using a developer that contains only a toner has been proposed so as to decrease the size of the apparatus (see, for example, JP 2001-356545 A). In such a configuration, a development roller that develops an electrostatic latent image by allowing a toner to adhere onto a circumferential surface of the photosensitive drum is disposed above a cleaning member of an upstream adjacently positioned photosensitive drum, thereby shortening the pitch or distance between the adjacent photosensitive drums so as to decrease the overall size of the apparatus. That is, this configuration provides a narrow-pitch image forming system.
Generally, as the developing system, a two-component developing system using a developer containing a toner and a magnetic carrier that can provide high image quality and low operating costs has been widely used, in addition to the non-magnetic one-component developing system. In the two-component developing system, the developer is rubbed against a surface of the photosensitive drum with a magnetic brush by using the development roller that is provided with a development sleeve having magnets disposed therein and carries the developer. Only the toner is transferred onto the surface of the photosensitive drum, thus developing the electrostatic latent image.
In order to satisfy all of the demands for a decrease in size, high image quality and low operating costs, which have been further become more stringent recently, the present inventors have developed an image forming apparatus in which a narrow-pitch image forming system is provided with a two-component developing system.
In the two-component developing system, detaching the developer that is attached on the surface of the development roller after the development is important, and if the detachment of the developer, after development, is not sufficient, a toner density of the developer at a part where the toner is consumed and a toner density where the toner is not consumed are different, which may cause generation of unevenness of density, which is called a “ghost (or memory)” image. Generally, such detachment of a developer is performed by: providing an odd number of magnets in the development roller; disposing a pair of magnets having a same pole in a position below a shaft center (a rotational center line) of the development roller so as to provide a detaching region where a magnetic force is substantially zero; and dropping the developer, after development, by free-fall using gravity in this region. Then, the detached developer is transported by a developer transporting screw that is disposed near the detaching region, and is adjusted to have a predetermined toner density again by being circulated in the developing apparatus.
However, in the narrow-pitch image forming system in which the development roller is positioned above the cleaning member of the upstream adjacently positioned photosensitive drum, since the cleaning member of the upstream adjacently positioned photosensitive drum is positioned below and close to the development roller, the detaching region of the developer, where the pair of magnets of the same polarity are disposed is inevitably positioned above the shaft center (the rotational center line) of the development roller. As a result, the detachment of the developer using gravity cannot be achieved, which may cause generation of the unevenness in density which can give rise to the above-noted “ghost”.
In order to solve this problem, a configuration where a drawing-up roller having a magnet therein is disposed near the detaching region on the development roller, and the developer, after development, is detached by a magnetic force of the magnet has been proposed (see, for example, JP 11-65247 A). In this case, the detached developer is drawn up (or taken up) by another drawing-up roller, and thereafter is transported to a developer stirring chamber having a screw, where the toner density is adjusted again and the toner is electrified.
However, in the above-discussed configuration where the detaching region of the developer, in which the pair of magnets of the same polarity are disposed, is positioned above the shaft center (the rotational center line) of the development roller, when the developer, after development, is detached by using the drawing-up roller having the magnet therein as described in JP 11-65247 A, the developer stirring chamber or the like for stirring the developer must be provided separately. Thus, a problem arises in that the developing apparatus is complicated in configuration and has a large size. In addition, the magnet is required to be disposed in the drawing-up roller, which may increase manufacturing costs and a weight of the apparatus.